The invention relates to test and measurement and data acquisition systems, and more particularly to signal conditioning and switching systems and circuitry for conditioning and routing signals.
Scientists and engineers often use test and measurement and data acquisition systems to perform a variety of functions, including laboratory research, process monitoring and control, data logging, analytical chemistry, test and analysis of physical phenomena and control of mechanical or electrical machinery, to name a few examples. One example of hardware to implement such measuring systems is a computer-based measurement system or data acquisition (DAQ) system. A computer-based measurement or DAQ system typically includes transducers for measuring and providing electrical signals, signal conditioning hardware which may perform amplification, isolation and/or filtering, and measurement or DAQ hardware for receiving digital and analog signals and providing them to a processing system, such as a personal computer. The computer may further include analysis hardware and software for analyzing and appropriately displaying the measured data.
As mentioned above, a measurement system may include one or more of a measurement or DAQ device comprised in or connected to a computer system. The device may be an I/O board plugged into one of the I/O slots of the processing or computer system. The measurement or DAQ device may also comprise an external device connected to a computer system. Exemplary hardware I/O (input/output) interface options include the GPIB (general purpose interface bus), the VXI bus, the PXI bus, or a serial bus such as the RS-232 protocol, IEEE 1394, or USB.
PC based measurement and data acquisition systems are used in a wide range of applications in the laboratory, in the field, and on the manufacturing plant floor. Typically, measurement or DAQ devices, such as DAQ plug-in boards, are general purpose data acquisition instruments that are well suited for measuring voltage signals. However, most real world sensors and transducers generate signals that must be conditioned before a measurement or DAQ device can reliably and accurately acquire the signal. This front-end processing is referred to as signal conditioning. Most PC based measurement and DAQ systems include some form of signal conditioning in addition to the measurement or DAQ device, usually a DAQ plug-in board in a personal computer.
Signal conditioning includes functions such as signal amplification, filtering, electrical isolation, linearization, transducer excitation, switching and multiplexing. Amplification is one of the most common types of signal conditioning. For example, low level thermocouple signals typically must be amplified to increase their resolution. Transducer excitation refers to signal conditioning hardware creating excitation for certain transducers such as strain gauges and RTDs, which require external excitation voltages or currents. Signal conditioning hardware also may linearize voltage levels from transducers so that a simple scaling function can convert the voltage to the measured phenomena. Isolation is another common use for signal conditioning hardware in order to isolate the transducer signals from the computer for safety purposes. The unit under test or device being monitored may contain high voltage transients that can damage the computer or harm the operator. Isolation is used to remove the possibility of these large voltage spikes affecting the computer. Filtering involves the signal conditioning hardware filtering unwanted signals. For example, the signal conditioning peripherals can contain low pass filters that eliminate high frequencies that can produce erroneous data. A common use of a low pass filter is one that filters out the 60 Hz AC power line noise present in most laboratories and factories.
As mentioned above, signal conditioning systems may also perform a switching or multiplexing function. Front-end switching systems increase the functionality of the measurement and automation system. General purpose switching delivers digital control of the presence or absence of a signal in the system. Multiplexers/matrix relay configurations control source and signal routing for the system, as well as act as a multiplexing front end for devices such as digital multimeters.
There are various types or classes of signal conditioning hardware. For example, the signal conditioning circuitry may be comprised directly on the measurement device or data acquisition device or board. This has advantages of lower cost and a requirement of less physical space. However, this also has numerous disadvantages such as no isolation of signals, low channel count, inability to perform distributed or remote I/O, and a lack of modularity, possibly requiring a different board for each type of transducer.
An alternative method for performing signal conditioning involves a direct connect modular system which may include a signal conditioning module for each channel. A direct connect modular system typically comprises a two port system that receives unconditioned signals on one port and sends out the conditioned signals on a second port to a measurement or DAQ device or board in the computer system. An advantage of this system is that the signals are conditioned outside of the noisy computer environment. Also, the system provides modularity, and developers can configure each channel for different transducer types. However, this two-port system generally cannot perform distributed or remote I/O and has a low channel count.
National Instruments Corporation developed a signal conditioning system referred to as Signal Conditioning eXtensions for Instrumentation (SCXI), which is a high-performance multi-channel signal conditioning front end system for PC based measurement devices and DAQ boards. An SCXI system comprises a rugged low noise external chassis that houses signal conditioning modules for amplifying, multiplexing and isolating field signals. SCXI modules are inserted into the SCXI chassis and perform various signal conditioning functions. The SCXI modules are fully shielded signal conditioning circuit boards that plug directly into the SCXI backplane/power bus. The SCXI includes guarded analog and digital buses and complete shielding and thus is able to handle sensitive analog measurements. As a result, when it is desirable to connect high level amplified signals to the noisy PC environment, transducer leads and signals are connected to shielded terminal blocks that plug directly on to signal conditioning modules.
The SCXI system expanded prior art external signal conditioning systems from a two port configuration to a three port configuration. The third port in the SCXI system is a bus backplane with a guarded analog bus, digital control, timing signals, and trigger signals. The bus backplane solves the low channel count limitation because the system can multiplex thousands of signals on to the analog bus. The conditioned signals may then be connected to the measurement device or DAQ board, which digitizes the conditioned signals. The third port also makes the SCXI system more flexible than a two port system because the user can plug modules such as digitizers into the bus and use the system for distributed I/O.
An SCXI system is designed to work with a wide range of applications and in a wide range of industries. An SCXI system is especially good for large channel count production or industrial test or automation systems that require a variety of transducers and signal types. SCXI modules can be easily reconfigured for separate types of signals or transducers, including thermocouples, RTDs, strain gauges, various filtering sources, current sources, and digital signals.
However, current signals conditioning systems, such as the SCXI signal conditioning system, lack the ability to switch high voltage analog signals. This limits the ability of the signal conditioning system with respect to high voltage signals. Therefore, an improved signal conditioning and switching system is desired which provides high voltage signal conditioning capabilities, including switching and multiplexing capabilities.
The present invention comprises a signal conditioning system and switching which includes a high voltage analog bus according to the present invention. The signal conditioning system comprises a chassis having a plurality of slots, wherein each of the slots is adapted to receive a module. The module may be a signal conditioning module, including a switching module, or an instrument module.
The chassis includes a low voltage analog bus (or backplane) comprised in the chassis and adapted for transmitting low voltage electrical signals. The low voltage analog bus includes one or more analog channels and is preferably the SCXI backplane. The chassis also includes a high voltage analog bus (or backplane) comprised in the chassis and adapted for transmitting high voltage electrical signals. The high voltage analog bus allows for the conditioning (including switching) of high voltage electrical signals.
The chassis further includes a plurality of connectors positioned in the chassis and electrically coupled to one or more of the low voltage analog bus and the high voltage analog bus. Each connector is adapted to connect a module to one or more of the low voltage analog bus and the high voltage analog bus. Thus, modules may be placed in slots of the chassis, wherein each module may connect to one or both of the low voltage analog bus and the high voltage analog bus.
In one embodiment, the plurality of connectors includes a plurality of low voltage connectors adapted to connect a module to the low voltage analog bus and a plurality of high voltage connectors adapted to connect a module to the high voltage analog bus. Each module may have one or more corresponding or mating connectors for connecting to one or both of the low voltage analog bus and the high voltage analog bus. Thus the signal conditioning system may include a low voltage module comprised in a slot of the chassis which connects through a low voltage connector to the low voltage analog bus. In a similar manner, the signal conditioning system may include a high voltage module comprised in a slot of the chassis which connects through a high voltage connector to the high voltage analog bus. The signal conditioning system may further include a safety switch, e.g., comprised on an instrument module, which is software programmable to connect a module to either the low voltage analog bus or the high voltage analog bus. In an alternate embodiment, a slot of the chassis may include only a low voltage connector for connecting a module to only the low voltage analog bus in that slot, and/or a slot of the chassis may include only a high voltage connector for connecting a module to only the high voltage analog bus in the respective slot.
The signal conditioning system may further have an external connector comprised on the chassis which is electrically connected to one or more of the low voltage analog bus and the high voltage analog bus. The external connector may be adapted to connect to an external instrument. Thus an external instrument may connect through the external connector to one or both of the low voltage analog bus and the high voltage analog bus.
In one embodiment, a module may include an instrument connector, preferably on the front of the module. When the module is inserted into a slot of the chassis, the instrument connector on the module may be used to connect an external instrument through the module to one or both of the low voltage analog bus and the high voltage analog bus. The chassis may also include an external connector which may electrically connect to a module inserted into a slot of the chassis, e.g., an instrument module, wherein the external connector is adapted for connecting the module to one or both of the low voltage analog bus and the high voltage analog bus.
In an alternate embodiment of the invention, the signal conditioning system may comprise a chassis which only includes a low voltage analog bus, i.e., the chassis does not include a built-in high voltage analog bus. For example the signal conditioning system may be an existing SCXI chassis which does not include a high voltage analog bus. In this embodiment, the system may include one or more rear connectors which are adapted to connect to one or more of the modules comprised in the chassis. The one or more rear connectors form a high voltage analog bus for transmitting high voltage electrical signals between the one or more modules.